Electrolytic capacitors have an advantage that they have large capacitance despite a small size, and therefore they are widely used in a low frequency filter and for by-pass The electrolytic capacitors generally have a structure such that an anode foil and a cathode foil intermediated by a separator are spirally wound together and placed and sealed in a casing (see FIGS. 1 and 2). As an anode foil, a valve metal such as aluminum or tantalum, having an insulating oxide film thereon as a dielectric layer is generally used, and as a cathode foil, an etched aluminum foil is generally used. For preventing short-circuiting between the anode and the cathode, the separator disposed therebetween is impregnated with an electrolyte, and it functions as a true cathode. Thus, the electrolyte is an important constituent which largely affects the properties of the electrolytic capacitor.
In the properties of the electrolyte, electric conductivity directly affects the energy loss and impedance properties of the electrolytic capacitor, and therefore, studies are being made intentionally on development of an electrolyte having high electric conductivity. For example, International Patent Publication No. WO95/15572 and Japanese Prov. Patent Publication No. 283379/1997 propose an electrolyte comprising a quaternary amidinium salt of phthalic acid or maleic acid dissolved in an aprotic solvent such as γ-butyrolactone. The electrolyte of this type has high electric conductivity; however, it has a problem that withstand voltage property is low. Generally, it is necessary that an electrolyte for an electrolytic capacitor should have higher withstand voltage than the rated voltage thereof. The electrolyte using a quaternary amidinium salt of phthalic acid or maleic acid can be used only in an electrolytic capacitor at rated voltage of 35 V or lower.
For solving the above problem, Japanese Prov. Patent Publication No. 135081/1998 proposes an electrolyte comprising a quaternary amidinium salt of benzoic acid as a solute and an organosol of metal oxide particles such as silica. This electrolyte has both high electric conductivity and high withstand voltage property, and hence, can be used in an electrolytic capacitor at rated voltage of up to 100 V. However, this electrolyte has unsatisfactory thermal stability and therefore, it cannot be used in an electrolytic capacitor operated in a high temperature environment at 125° C. or higher.